Golf ball

ABSTRACT

A golf ball includes a core and a cover. The core is formed of a material molded under heat from a rubber composition. The rubber composition includes components (A) through (C). The components (A) through (C) are (A) a base rubber, (B) an organic peroxide, and (C) a metal salt containing hydration water.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority to Japanese patentapplication No. 2018-124750, filed on Jun. 29, 2018, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to golf balls.

2. Description of the Related Art

It has been known that when striking a golf ball with a driver or thelike, a lower (smaller) amount of spin on the golf ball leads to alonger distance and is thus significant. Therefore, golf balls that spinat lower rate when struck by a driver or the like have been studied.

For example, Japanese Laid-open Patent Publication No. 2015-47502 (JP2015-47502) describes a golf ball that includes a core and a coverhaving one or more layers, in which the core is formed of a materialmolded under heat from a rubber composition including the followingcomponents (A) through (C):

(A) a base rubber,(B) an organic peroxide, and(C) water and/or a metal monocarboxylate.

According to the golf ball described in JP 2015-47502, it is possible toobtain a core material that is limited in resilience decrease over timeand in energy loss, so that it is possible to maintain good resilienceand reduce spin rate to increase distance.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a golf ball includes acore and a cover. The core is formed of a material molded under heatfrom a rubber composition. The rubber composition includes components(A) through (C). The components (A) through (C) are (A) a base rubber,(B) an organic peroxide, and (C) a metal salt containing hydrationwater.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and notrestrictive of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Poor workability or kneadability in including water in a rubbercomposition may result in insufficient durability of a golf ball madeusing the rubber composition.

According to an aspect of the present invention, a low spin golf ballwith durability is provided.

Specific examples of golf balls according to an embodiment of thepresent invention are described below with reference to the accompanyingdrawings.

A golf ball according to this embodiment includes a core and a cover,where the core is formed of a material molded under heat from a rubbercomposition including the following components (A) through (C):

(A) a base rubber,(B) an organic peroxide, and(C) a metal salt containing hydration water.

As described above, a golf ball according to this embodiment may includea core and a cover. First, components included in a rubber compositionused in forming the core are described below.

(A) Base Rubber

The base rubber is not limited in particular, and various rubbermaterials may be used. It is preferable, however, to use polybutadiene(polybutadiene rubber) in particular.

Polybutadiene used for the base rubber has a cis-1,4-bond content ofpreferably 60% or more, more preferably, 80% or more, still morepreferably, 90% or more, and particularly preferably, 95% or more, inits polymer chain. It is possible to prevent reduction in resilience byincreasing the proportion of cis-1,4-bonds to the bonds in polybutadienemolecules.

The 1,2-vinyl bond content of the polybutadiene, which is not limited inparticular, is preferably 2% or less, more preferably, 1.7% or less, andstill more preferably, 1.5% or less, in its polymer chain. It ispossible to prevent reduction in resilience by reducing the 1,2-vinylbond content.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably20 or more, and more preferably, 30 or more. The upper limit of theMooney viscosity, which is not limited in particular, either, ispreferably 120 or less, more preferably, 100 or less, and still morepreferably, 80 or less.

The Mooney viscosity is an industrial viscosity index (JIS [JapaneseIndustrial Standards] K 6300-1 (2013)) measured with a Mooneyviscometer, which is a type of rotary plastometer, and uses ML₁₊₄ (100°C.) as a unit symbol, where M stands for Mooney viscosity, L stands fora large rotor (L-type), 1+4 stands for a pre-heating time of one minuteand a rotor rotation time of four minutes, and 100° C. indicates thatmeasurement is conducted under a condition of 100° C.

The polybutadiene may be synthesized using a rare-earth catalyst or agroup VIII metal compound catalyst. In particular, polybutadienesynthesized using a rare-earth catalyst may be suitably used.Furthermore, these catalysts may be used in combination with one or moreof an organoaluminum compound, an alumoxane, a halogen-containingcompound, and a Lewis base on an as-needed basis. As the compoundsillustrated above, those described in Japanese Laid-open PatentPublication No. 11-35633 may be suitably used.

In the case of synthesizing polybutadiene, it is particularly preferableto use a neodymium (Nd) catalyst that uses a neodymium compound, whichis a lanthanum series rare-earth compound, among rare-earth catalysts.In this case, it is possible to obtain polybutadiene having a highcis-1,4-bond content and a low 1,2-vinyl bond content with excellentpolymerization activity.

The base rubber may include polybutadiene rubber synthesized with acatalyst different from the lanthanum series rare-earth compound. Thebase rubber may alternatively include, for example, styrene-butadienerubber (SBR), natural rubber, polyisoprene rubber, orethylene-propylene-diene rubber (EPDM). These rubbers may be used aloneor in any combination.

(B) Organic Peroxide

The organic peroxide, which is not limited in particular, is preferablyan organic peroxide having a one-minute half-life temperature of 110° C.to 185° C. One or more types of organic peroxides may be used alone orin combination.

The amount of inclusion of the organic peroxide, which is not limited inparticular, is preferably 0.1 parts by mass or more, and morepreferably, 0.3 parts by mass or more, per 100 parts by mass of the baserubber.

The upper limit of the amount of inclusion of the organic peroxide,which is not limited in particular, either, is preferably 5 parts bymass or less, more preferably, 4 parts by mass or less, and still morepreferably, 3 parts by mass or less, per 100 parts by mass of the baserubber. As the organic peroxide, commercially available products may beused, whose examples include those available under the trade names ofPercumyl D, Perhexa C-40, Niper BW and Peroyl L (all manufactured by NOFCorporation), and Luperco 231XL (manufactured by Atochem Co.).

(C) Metal Salt Containing Hydration Water

As disclosed in JP 2015-47502, it has been attempted to reduce the spinrate of golf balls obtained by including water in a rubber compositionfor forming a core.

It is known that the decomposition efficiency of an organic peroxide ina rubber composition changes with temperature, and the decompositionefficiency increases as the temperature increases from a certaintemperature. If the temperature is too high, the amount of radicals intowhich the organic peroxide is decomposed becomes too large, thusresulting in recombination and deactivation of radicals. As a result,fewer radicals act effectively for crosslinking.

When the organic peroxide decomposes to generate decomposition heat atthe time of core vulcanization, the temperature near the surface of thecore is kept substantially the same as the temperature of avulcanization mold, while the temperature near the center of the corebecomes considerably higher than the mold temperature because of theaccumulated decomposition heat of the organic peroxide that hasdecomposed from the outer side.

In the case of including water in the rubber composition, water isbelieved to serve as a promoter of the decomposition of the organicperoxide, and it is inferred that the inclusion of water in the rubbercomposition causes the above-described radical reaction to differbetween the center and the surface of the core. That is, the inclusionof water in the rubber composition is believed to further promote thedecomposition of the organic peroxide to further promote thedeactivation of radicals, thereby further reducing the effective amountof radicals, near the center of the core. Therefore, it is possible toobtain a core whose crosslinking density differs between the surface andthe center, and to obtain a core having different dynamic viscoelasticproperties at its central portion. By providing a golf ball with such acore, it is possible to reduce the spin rate of the golf ball.

As described above, however, poor workability or kneadability inincluding water in a rubber composition may result in insufficientdurability of a golf ball that is made using the rubber composition.Furthermore, part of water may evaporate during the kneading of therubber composition. Therefore, water is not included as desired in partof the rubber composition.

JP 2015-47502 illustrates that a metal monocarboxylate can substitutefor water. The metal monocarboxylate, however, generates and supplieswater through a dehydration condensation reaction between two molecules,and therefore, the reaction does not progress uniformly. Accordingly,even when water is substituted with a metal monocarboxylate, a golf ballmade using the rubber composition may not have sufficient durability.

Therefore, the inventors of the present invention have made an earneststudy of golf balls having low spin rate and durability. As a result,the inventors have discovered, to complete the present invention, that agolf ball having both low spin rate and durability can be obtained byusing a metal salt containing hydration water instead of water as awater source, to increase workability and kneadability.

By using a metal salt containing hydration water, which is solid,instead of water, which is liquid, as a water source as described above,it is possible to improve workability and kneadability, and inparticular, to evenly disperse the metal salt containing hydration waterthat is a water source in the rubber composition. Furthermore, hydrationwater is released, for example, at a temperature near or higher than100° C. Therefore, unlike in the case of using water, it is possible toprevent water from being released during the kneading of the rubbercomposition. Furthermore, because hydration water can be releasedthrough no intermolecular reaction, it is possible to uniformly generatewater in accordance with vulcanization temperature.

Therefore, water can be supplied uniformly and included as desired inthe rubber composition during vulcanization, and it is possible toobtain a golf ball having both low spin rate and durability by using therubber composition including the metal salt containing hydration water.

The metal salt containing hydration water is not limited to a particularkind, and various kinds of metal salts containing hydration water may beused.

In terms of increasing water supply efficiency and reducing a metal saltremaining after supplying water, however, it is preferable to use ametal salt containing hydration water having a high moisture percentageas percentage by mass in the molecular formula.

Specifically, the moisture content of the metal salt containinghydration water in its molecular formula is preferably 6% by mass ormore, and more preferably, 15% by mass or more.

A higher moisture content of the metal salt containing hydration waterin its molecular formula is more preferable. The upper limit of themoisture content, which is not limited in particular, may be, forexample, 90% by mass or less in terms of, for example, availability.

Preferably, the metal salt containing hydration water can release asmuch water as possible when vulcanizing the rubber composition. Asdescribed below, the rubber composition is vulcanized at, for example,approximately 100° C. to approximately 200° C.

Therefore, the dissociation rate of water of the metal salt containinghydration water in the case of heating the metal salt up to 155° C.,namely, the cumulative dissociation rate of water in the case of heatingthe metal salt up to 155° C., is preferably 60% by mass or more. Becausethe entirety of water is preferably dissociated during vulcanization,the dissociation rate of water in the case of heating the metal salt upto 155° C. is preferably 100% or less.

Furthermore, the dissociation rate of water of the metal salt containinghydration water during the kneading of the rubber composition ispreferably low. Therefore, for example, the dissociation rate of waterin the case of heating the metal salt up to 90° C., namely, thecumulative dissociation rate of water in the case of heating the metalsalt up to 90° C., is preferably 60% by mass or less.

The metal salt containing hydration water, which is not limited inparticular, is preferably an inorganic compound, which has excellentstorage stability.

As the metal salt containing hydration water, specifically, one or moreselected from, for example, calcium sulfate 0.5-hydrate, calcium sulfatedihydrate, aluminum sulfate 14-18-hydrate, magnesium sulfateheptahydrate, beryllium sulfate tetrahydrate, zirconium sulfatetetrahydrate, manganese sulfate pentahydrate, iron sulfate heptahydrate,cobalt sulfate heptahydrate, nickel sulfate hexahydrate, cupric sulfatepentahydrate, zinc sulfate heptahydrate, cadmium sulfate octahydrate,indium sulfate nonahydrate, and zinc sulfate dihydrate may be preferablyused. Two or more of those may be used in mixture as the metal saltcontaining hydration water.

As the metal salt containing hydration water, more preferably, one ormore selected from calcium sulfate 0.5-hydrate, calcium sulfatedihydrate, aluminum sulfate 14-18-hydrate, and magnesium sulfateheptahydrate may be used in terms of availability and the masspercentage of contained hydration water.

Table 1 shows the moisture (hydration water) content (mass percentage)of calcium sulfate 0.5-hydrate, calcium sulfate dihydrate, aluminumsulfate 14-hydrate, and magnesium sulfate heptahydrate in molecularformula as moisture percentage. Furthermore, Table 1 also shows thecumulative dissociation rate of water up to each temperature aspercentage by mass in the case of heating the above-described metalsalts containing hydration water.

The cumulative dissociation rate of water was calculated from a the/malweight loss curve obtained by measuring an evaluation sample of 10 mg inamount from room temperature up to 300° C. with a temperature rise rateof 5° C./min. in a nitrogen flow of 150 mL/min., using a TG-DTAapparatus (model: TG8121, manufactured by Rigaku Corporation) withα-Al₂O₃ manufactured by Rigaku Corporation used as a referencesubstance.

The same samples as evaluated in Table 1 are used in the followingexamples.

TABLE 1 Moisture Cumulative dissociation rate of percentage water up toeach temperature (%) (%) 90° C. 120° C. 155° C. Calcium 6.21 25.6 96.398.1 sulfate 0.5- hydrate Calcium 20.93 0.0 60.0 93.4 sulfate dihydrateAluminum 42.43 13.9 45.6 60.4 sulfate 14- hydrate Magnesium 51.16 55.278.4 89.4 sulfate heptahydrate

The above-described rubber composition may further include othercomponents as desired than the above-described components (A) through(C).

The rubber composition may include components such as (D) anorganosulfur compound, (E) a co-crosslinking agent, and (F) an inertfiller as desired, and may also include (G) an antioxidant on anas-needed basis. These optional additive components are described indetail below.

(D) Organosulfur Compound

The above-described rubber composition may include an organosulfurcompound. Examples of this organosulfur compound, which is not limitedin particular, include thiophenols, thionaphthols, halogenatedthiophenols, and metal salts thereof, and more specifically, includezinc salts of pentachlorothiophenol, pentafluorothiophenol,pentabromothiophenol and parachlorothiophenol, and diphenylpolysulfides,dibenzylpolysulfides, dibenzoylpolysulfides,dibenzothiazoylpolysulfides, and dithiabenzoylpolysulfides having 2 to 4sulfurs. These may be used alone or in any combination. Of these, one ormore selected from a zinc salt of pentachlorothiophenol anddiphenyldisulfide may be suitably used.

By including an organosulfur compound in the rubber composition, it ispossible to improve the resilience of a resulting golf ball.

The amount of inclusion of the organosulfur compound, which is notlimited in particular, is preferably 0.1 parts by mass or more, morepreferably, 0.2 parts by mass or more, and still more preferably, 0.5parts by mass or more, per 100 parts by mass of the base rubber.

The upper limit of the amount of inclusion of the organosulfur compound,which is not limited in particular, either, is preferably 5 parts bymass or less, more preferably 4 parts by mass or less, and still morepreferably, 3 parts by mass or less, per 100 parts by mass of the baserubber. By determining the range of the amount of inclusion of theorganosulfur compound as described above, it is possible to prevent thematerial molded under heat from the rubber composition from becoming toosoft.

(E) Co-Crosslinking Agent

As the co-crosslinking agent, a metal salt of an α,β-unsaturatedcarboxylic acid having 3 to 8 carbons may be preferably used. Examplesof α,β-unsaturated carboxylic acids include acrylic acid, methacrylicacid, maleic acid, and fumaric acid, of which acrylic acid, whichprovides high resilience, is preferable. Example metals of the metalsalt include zinc, sodium, magnesium, calcium, and aluminum, of whichzinc is particularly preferable. Accordingly, as the co-crosslinkingagent, zinc acrylate can be suitably used.

The amount of inclusion of the co-crosslinking agent, which is notlimited in particular, is preferably 3 parts by mass or more and 60parts by mass or less per 100 parts by mass of the base rubber. Byincluding 3 parts by mass or more of the co-crosslinking agent per 100parts by mass of the base rubber, it is possible to increase resilience.By including 60 parts by mass or less of the co-crosslinking agent per100 parts by mass of the base rubber, it is possible to prevent thematerial molded under heat from the rubber composition from becoming toohard and thus to improve the impact feel of a golf ball.

The amount of inclusion of the co-crosslinking agent is more preferably5 parts by mass or more and 45 parts by mass or less per 100 parts bymass of the base rubber.

(F) Inert Filler

As the inert filler, one or more inorganic fillers selected from, forexample, zinc oxide, barium sulfate, and calcium carbonate may be used.

By adding the inert filler, it is possible to adjust the initialvelocity and the specific gravity of a golf ball.

In the case of adding the inert filler, the amount of inclusion of theinert filler is preferably 1 part by mass or more, more preferably, 3parts by mass or more, and still more preferably, 5 parts by mass ormore, per 100 parts by mass of the base rubber.

The upper limit of the amount of inclusion of the inert filler, which isnot limited in particular, either, is preferably 100 parts by mass orless, more preferably, 60 parts by mass or less, still more preferably,45 parts by mass or less, and particularly preferably, 40 parts by massor less, per 100 parts by mass of the base rubber.

(G) Antioxidant

As described above, the rubber composition may include an antioxidant onan as-needed basis.

Examples of the antioxidant include commercially available products suchas Nocrac NS-6, Nocrac NS-30, and Nocrac 200 (all manufactured by OuchiShinko Chemical Industry Co., Ltd.). These may be used alone or in anycombination.

In the case of adding the antioxidant, the amount of inclusion of theantioxidant is not limited in particular, but is preferably 0.1 parts bymass or more per 100 parts by mass of the base rubber.

In the case of adding the antioxidant, the upper limit of the amount ofinclusion of the antioxidant is not limited in particular, either, butis preferably 5.0 parts by mass or less, more preferably, 4.0 parts bymass or less, and still more preferably, 3.0 parts by mass or less, per100 parts by mass of the base rubber.

By including 0.1 parts by mass or more and 5.0 parts by mass or less ofthe antioxidant per 100 parts by mass of the base rubber, an appropriatecore hardness gradient can be obtained in particular, and resilience,durability, and the spin reducing effect on full shots can be improvedin particular.

The core of the golf ball according to this embodiment may be obtainedby kneading the above-described rubber composition and vulcanizing andcuring the rubber composition in the same manner as known rubbercompositions for golf balls. As a condition of vulcanization, forexample, the vulcanization temperature is preferably 100° C. or higherand 200° C. or lower, more preferably, 135° C. or higher and 175° C. orlower, and still more preferably, 145° C. or higher and 165° C. orlower.

The vulcanization time, which is not limited in particular, either, maybe, for example, 2 minutes or more and 90 minutes or less.

The core diameter is preferably 30 mm or more, more preferably, 32 mm ormore, and still more preferably, 34 mm or more. The core diameter of 30mm or more makes it possible to improve the spin reducing effect inparticular and also to increase resilience.

The core diameter is, for example, preferably 40 mm or less, and morepreferably, 39 mm or less. The core diameter of 40 mm or less makes itpossible to improve the spin reducing effect in particular and also toensure a sufficient thickness of the cover. Therefore, it is possible tofurther provide an effect due to the cover.

Furthermore, the deflection hardness of the core (heat-molded material),which is the amount of deflection of the core between the state where aninitial load of 98 N (10 kgf) is applied on the core and the state wherea final load of 1275 N (130 kgf) is applied on the core, is not limitedin particular, but is preferably 2.5 mm or more, more preferably, 2.8 mmor more, and still more preferably, 3.0 mm or more.

By causing the core to have a deflection hardness of 2.5 mm or more, itis possible to improve the spin reducing effect in particular.

The upper limit of the deflection hardness of the core, which is notlimited in particular, is preferably 8.0 mm or less, more preferably,7.8 mm or less, and still more preferably, 7.5 mm or less.

By causing the core to have a deflection hardness of 8.0 mm or less, itis possible to improve the spin reducing effect in particular, and alsoto increase resilience.

Next, the hardness of the core is described.

The value of a JIS-C hardness difference obtained by subtracting thecenter hardness of the core from the surface hardness of the core [(coresurface hardness)−(core center hardness)] is not limited in particular,but is preferably 20 or more.

The JIS-C hardness difference of 20 or more makes it possible to improvethe spin reducing effect in particular.

The upper limit of the JIS-C hardness difference, which is not limitedin particular, either, is preferably 40 or less, and more preferably, 35or less.

The JIS-C hardness difference of 40 or less makes it possible toincrease the initial velocity and the distance of an actually struckgolf ball. Furthermore, it is possible to increase durability tobreakage due to repeated striking, in particular.

Here, the center hardness means the hardness measured at the center of across section obtained by cutting the core in half through its center,and the surface hardness means the hardness measured at the surface(spherical surface) of the core. Furthermore, the JIS-C hardness meansthe hardness measured with a (JIS-C type) spring-type hardness tester asspecified in JIS K 6301 (1975).

Next, the cover covering the core is described.

The material of the cover is not limited in particular. Known materialssuch as various ionomer resins and urethane elastomers used in golfballs may be used.

To achieve a golf ball further reduced in spin rate, it is particularlypreferable to use a highly neutralized ionomer material for the layernext to the core.

Specifically, it is preferable to use a mixture material that includes:a resin component including (a) a base resin and (b) a non-ionomericthermoplastic elastomer; (c) one or more selected from fatty acidshaving a molecular weight of 228 to 1500 and derivatives thereof; and(d) a basic inorganic metal compound capable of neutralizingunneutralized acid groups in the components (a) and (c).

The base resin (a) may be formulated by blending (a-1) one or moreselected from an olefin-unsaturated carboxylic acid binary randomcopolymer and a metal ion neutralizer of an olefin-unsaturatedcarboxylic acid binary random copolymer and (a-2) one or more selectedfrom an olefin-unsaturated carboxylic acid-unsaturated carboxylic acidester ternary random copolymer and a metal ion neutralizer of anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterternary random copolymer in the mass ratio of 100:0 to 0:100.

It is preferable to blend 5 parts by mass or more and 80 parts by massor less of the component (c) and 0.1 parts by mass or more and 17 partsby mass or less of the component (d), per 100 parts by mass of the resincomponent formulated by blending the base resin (a) and thenon-ionomeric thermoplastic elastomer (b) in the mass ratio of 100:0 to50:50.

In particular, in the case of using the mixture material of thecomponents (a) through (d), it is preferable that 70% or more of theacid groups of the mixture material be neutralized.

For the material of the outermost layer of the cover, it is preferableto use a urethane material, particularly, a thermoplastic urethaneelastomer, as a main material.

Furthermore, one or more cover layers (intermediate layers) may beformed between the layer next to the core and the outermost cover layer.In this case, it is preferable to use a thermoplastic resin such as anionomer as the material of the intermediate layers.

To obtain the cover of the golf ball according to this embodiment, forexample, it is possible to employ a method by which a core manufacturedin advance in accordance with a ball type is placed in a mold and theabove-described mixture mixed and melted under heat is provided aroundthe core by injection molding to coat the core with a desired cover. Inthis case, the cover can be manufactured with excellent thermalstability, fluidity, and moldability. As a result, the finally obtainedgolf ball has high resilience, good impact feel, and good scuffresistance. Cover forming methods that may be employed other than theone described above include, for example, a method by which a pair ofhemispheric half cups is molded in advance with the above-describedcover material, the core is enclosed in the half cups, and pressuremolding is performed on the half cups enclosing the core for 1 to 5minutes at 120° C. to 170° C.

When the cover has a single layer, the thickness of the cover ispreferably, for example, 0.3 mm or more and 3 mm or less. When the coverhas two layers, the thickness of the outer layer cover (outermost layercover) is preferably 0.3 mm or more and 2.0 mm or less and the thicknessof the inner layer cover (intermediate layer cover) is preferably 0.3 mmor more and 2.0 mm or less.

Furthermore, the Shore D hardness of each layer of the cover (each coverlayer), which is not limited in particular, is preferably 40 or more,and more preferably, 45 or more. The upper limit of the Shore D hardnessof each cover layer, which is not limited in particular, either, ispreferably 70 or less, and more preferably, 65 or less.

Numerous dimples are formed on the surface of the outermost layer of thecover, and various kinds of treatment such as surface preparation,stamping, and painting may be further performed on the cover. Suchsurface treatment can be performed on the cover famed of theabove-described cover material with good workability because the coversurface has good moldability.

The golf ball according to this embodiment uses the above-describedrubber composition as the material of at least one layer of the core,while the core may have two or more layers. When the core has two ormore layers, it is preferable to apply the core illustrated in thisembodiment to the innermost layer, but the present invention is notlimited to this. The golf ball is not limited to a particular type tothe extent that the golf ball includes the core and at least one coverlayer. Examples of golf ball types include two-piece or three-piecesolid golf balls having a solid core coated with a cover and solid golfballs such as multi-piece golf balls having a layer structure of threeor more layers.

EXAMPLES

The present invention is specifically described with reference toexamples and comparative examples shown below. The present invention,however, is not limited to the following examples.

Examples 1 to 5 and Comparative Examples 1 and 2 [Manufacture of Core]

The cores of Examples 1 to 5 and Comparative Examples 1 and 2 weremanufactured using the core materials including polybutadiene as aprincipal component as shown in Table 2 below.

The composition of the rubber composition was selected such that theobtained golf ball had an initial velocity of 77 m/s and a deflectionhardness of 2.40 mm. Furthermore, the specific gravity of the corecalculated from the composition is 1.132 g/cm³ in each of the examplesand comparative examples as shown in Table 4.

The details of the components of the core materials shown in Table 2 areas follows.

-   -   Polybutadiene rubber A: trade name “BR051” (manufactured by JSR        Corporation); a cis-1,4-bond content of 95%; a Mooney viscosity        [ML₁₊₄ (100° C.)] of 38; polymerized with a Nd catalyst    -   Polybutadiene rubber B: trade name “BR01” (manufactured by JSR        Corporation); a cis-1,4-bond content of 95%; a Mooney viscosity        [ML₁₊₄ (100° C.)] of 45; polymerized with a nickel (Ni) catalyst    -   Polybutadiene rubber C: trade name “BR730” (manufactured by JSR        Corporation); a cis-1,4-bond content of 95%; a Mooney viscosity        [ML₁₊₄ (100° C.)] of 56; polymerized with a Nd catalyst    -   Organic peroxide A: Dicumyl peroxide, trade name “Percumyl D”        (manufactured by NOF Corporation)    -   Organic peroxide B: a mixture of        1,1-di(t-butylperoxy)cyclohexane and silica, trade name “Perhexa        C-40”    -   Calcium sulfate 0.5-hydrate: calcined gypsum manufactured by        KANTO CHEMICAL CO., INC.    -   Calcium sulfate dihydrate: calcium sulfate dihydrate        manufactured by KANTO CHEMICAL CO., INC.    -   Aluminum sulfate 14-hydrate: aluminum sulfate 14-hydrate        manufactured by KANTO CHEMICAL CO., INC.    -   Magnesium sulfate heptahydrate: magnesium sulfate heptahydrate        manufactured by KANTO CHEMICAL CO., INC.    -   Water: distilled water manufactured by Wako Pure Chemical        Industries, Ltd.    -   Organosulfur compound: zinc salt of pentachlorothiophenol        manufactured by Wako Pure Chemical Industries, Ltd.    -   Zinc acrylate: manufactured by Nippon Shokubai Co., Ltd.    -   Barium sulfate: trade name “Barico #100” (manufactured by        Hakusui Tech)    -   Zinc oxide: trade name “Zinc Oxide Grade 3” (manufactured by        Sakai Chemical Co., Ltd.)    -   Antioxidant: trade name “Nocrac NS-6” (manufactured by Ouchi        Shinko Chemical Industry Co., Ltd.)

TABLE 2 Example 1 Example 2 Example 3 Example 4 Inner Polybutadiene — —— — layer A core Polybutadiene — — — — com- B position Polybutadiene100.00  100.00  100.00  100.00  (part by C mass) Organic 1.00 1.00 0.801.00 peroxide A Organic — — — — peroxide B Calcium 11.43  — — — sulfate0.5- hydrate Calcium — 4.09 5.10 — sulfate dihydrate Aluminum — — — 3.12sulfate 14- hydrate Magnesium — — — — sulfate heptahydrate Water — — — —Organosulfur 1.00 0.30 0.50 0.50 compound Zinc acrylate 41.41  43.30 43.04  39.48  Barium — — — — sulfate Zinc oxide 4.69 9.96 9.26 11.78 Antioxidant 0.10 0.10 0.10 0.10 Comparative Comparative Example 5Example 1 Example 2 Inner layer Polybutadiene A — 20.00  20.00  corePolybutadiene B — 80.00  80.00  composition Polybutadiene C 100.00  — —(part by Organic 1.00 0.60 1.00 mass) peroxide A Organic — 0.60 —peroxide B Calcium — — — sulfate 0.5- hydrate Calcium — — — sulfatedihydrate Aluminum — — — sulfate 14- hydrate Magnesium 1.75 — — sulfateheptahydrate Water — — 0.80 Organosulfur 0.80 0.30 0.35 compound Zincacrylate 40.01  29.80  42.96  Barium — 14.94  9.43 sulfate Zinc oxide12.38  4.00 4.00 Antioxidant 0.10 0.10 0.10

In manufacturing the core, first, of the components shown in Table 2,those other than the organic peroxides and the reagents for addingwater, specifically, calcium sulfate 0.5-hydrate, calcium sulfatedihydrate, aluminum sulfate 14-hydrate, magnesium sulfate heptahydrate,and water, were kneaded at or below a temperature of 120° C.

Next, one or both of the organic peroxides were added and one of thereagents for adding water was added (except for Comparative Example 1)to the resultant kneaded material, which was thereafter further kneadedat or below a temperature of 90° C. Thereby, the rubber composition ofeach of the examples and comparative examples was prepared.

The resultant mixture was vulcanized and molded under the conditions of155° C. and 19 minutes, thereby manufacturing the core.

[Manufacture of Golf Ball]

A cover formed of two layers, namely, the intermediate layer and theoutermost layer having respective thicknesses shown in Table 3 below,was formed on the surface of the manufactured core of each of theexamples and comparative examples, using resins having respectivecompositions shown in Table 3, thereby manufacturing golf balls.

The cover was formed by covering the core with the intermediate layerand the outermost layer in this order by injection molding, therebyobtaining multi-piece solid golf balls having a three-layer structure.At this point, although not graphically illustrated, common dimpleshaving a predetermined pattern were formed in the surface of the coverof the golf ball of each of the examples and comparative example.

The details of the components of the cover material shown in Table 3 areas follows.

-   -   Himilan 1706, Himilan 1557, and Himilan 1605: ionomer compounds        manufactured by Dupont-Mitsui Polychemicals Co., Ltd.    -   T-8283 and T-8290: urethane compounds manufactured by DIC-Bayer        Polymer, Ltd.    -   Hytrel 4001: a polyester elastomer manufactured by Dupont-Toray        Co., Ltd.    -   Polyethylene wax: trade name “SANWAX 161-P” manufactured by        Sanyo Chemical Industries, Ltd.    -   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate    -   Titanium oxide: “TIPAQUE R-680” manufactured by Ishihara Sangyo        Kaisha, Ltd.    -   rimethylolpropane: manufactured by Mitsubishi Gas Chemical        Company, Inc.

TABLE 3 Outermost Intermediate layer layer Thickness (mm) 0.82 1.20Composition Himilan 1706 — 35 (part by Himilan 1557 — 15 mass) Himilan1605 — 50 T-8290 75 — T-8283 25 — Hytrel 4001 11 — Silicon wax 0.6 —Polyethylene wax 1.2 — Isocyanate 7.5 — compound Titanium oxide 3.9 —Trimethylolpropane — 1.1

[Core Evaluation Method]

The cores manufactured in the examples and comparative examples wereevaluated for deflection hardness, outer diameter, weight, and JIS-Chardness difference according to the following procedure. The resultsare shown in Table 4.

[Deflection Hardness]

The amount of deformation of the core between the state where an initialload of 98 N (10 kgf) was applied on the core and the state where afinal load of 1275 N (130 kgf) was applied on the core was determined asthe measured value of a single core, and the average of the measuredvalues of thirty cores manufactured under the same conditions wasdetermined as the deflection hardness of the core for each of theexamples and comparative examples.

[Outer Diameter]

The core surface was measured at five random points at a temperature of23.9±1° C., and the average of the measured values was determined as themeasured value of a single core. The average of the measured values offive cores manufactured under the same conditions was determined as theouter diameter of the core for each of the examples and comparativeexamples.

[Weight]

The weight of the core was measured using a plate electronic scale.

[JIS-C Hardness Difference]

The core has a spherical surface. The probe of a hardness tester was setsubstantially perpendicular to the spherical surface to measure the coresurface hardness in JIS-C hardness according to the JIS K 6301 (1975)standard.

With respect to the center hardness of the core, the core was cut inhalf through its center by a fine cutter, and the center of the crosssection was measured in JIS-C hardness.

Then, the JIS-C hardness difference was calculated by subtracting themeasured core center hardness from the measured core surface hardness.

[Golf Ball Evaluation Method]

The golf balls manufactured in the examples and comparative exampleswere evaluated for deflection hardness, outer diameter, weight, spinrate, and durability performance according to the following procedure.The results are shown in Table 4.

[Deflection Hardness]

The amount of deformation (mm) of the golf ball between the state wherean initial load of 98 N (10 kgf) was applied on the golf ball and thestate where a final load of 1275 N (130 kgf) was applied on the golfball was measured and determined as the deflection hardness of the golfball with respect to each of the golf balls manufactured in the examplesand comparative examples.

[Outer Diameter]

The golf ball surface was measured at five random dimple-free points ata temperature of 23.9±1° C., and the average of the measured values wasdetermined as the measured value of a single golf ball. The average ofthe measured values of five golf balls manufactured under the sameconditions was determined as the outer diameter of the golf ball foreach of the examples and comparative examples.

[Weight]

The weight of the golf ball was measured using a plate electronic scale.

[Spin Rate]

A driver (W#1) “TourStage ViQ” (2012 model) manufactured by BridgestoneSports Co., Ltd. (loft angle of 11.5°) was mounted on a golf swingrobot, and the spin rate of the golf ball immediately after the golfball was struck by the driver at a head speed (HS) of 45 m/s wasmeasured with an apparatus for measuring the initial conditions.

Furthermore, a number 6 iron (I#6) “TourStage ViQ” (2012 model)manufactured by Bridgestone Sports Co., Ltd. was mounted on the golfswing robot, and the spin rate of the golf ball immediately after thegolf ball was struck by the number 6 iron at a head speed (HS) of 38 m/swas measured with the apparatus for measuring the initial conditions.

The spin rate is indicated by the difference from the measurement resultof Comparative Example 1 in which no water was added in manufacturingthe core. A number smaller than zero indicates a lower spin rate.

[Durability Performance]

The durability of the golf ball was evaluated using an ADC Ball CORDurability Tester manufactured by Automated Design Corporation (U.S.).This tester pneumatically launches a golf ball and thereafter causes thegolf ball to sequentially collide with two parallel metal plates. Thevelocity of incidence on the metal plates was 43 m/s.

The percentage of the number of golf balls cracked before being launchedone hundred times among one hundred golf balls manufactured in each ofthe examples and comparative examples in the case of launching each ofthe hundred golf balls one hundred times was calculated and determinedas durability performance (%). Therefore, the smaller the durabilityperformance (%), the lower the failure rate and the better thedurability of the golf ball.

TABLE 4 Example 1 Example 2 Example 3 Example 4 Core Specific 1.1321.132 1.132 1.132 gravity (g/cm³) Deflection 3.12 3.18 2.99 3.01hardness (mm) Outer diameter 38.71 38.51 38.53 38.60 (mm) Weight (g)35.27 34.61 34.71 34.76 Surface 32.5 32.5 38.1 25.7 hardness- centerhardness (JIS-C) Golf Deflection 2.40 2.40 2.40 2.40 ball hardness (mm)Outer diameter 42.69 42.70 42.70 42.71 (mm) Weight (g) 45.48 45.34 45.3845.33 Initial 77 77 77 77 velocity (m/s) Spin Driver −56 −62 −122 −36rate W#1 (rpm) Iron I#6 −243 −253 −402 −168 Durability 3.5 2.5 8.1 4.2performance (%) Comparative Comparative Example 5 Example 1 Example 2Core Specific 1.132 1.132 1.132 gravity (g/cm³) Deflection 2.87 2.813.10 hardness (mm) Outer diameter 38.59 38.65 38.57 (mm) Weight (g)34.85 34.96 34.81 Surface 27.9 19.0 29.8 hardness- center hardness(JIS-C) Golf Deflection 2.40 2.40 2.40 ball hardness (mm) Outer diameter42.70 42.69 42.71 (mm) Weight (g) 45.40 45.42 45.47 Initial 77 77 77velocity (m/s) Spin Driver −77 0 −165 rate W#1 (rpm) Iron I#6 −218 0−389 Durability 7.5 0.0 11.0 performance (%)

From the results shown in Table 4, in Examples 1 to 5, in which therubber composition includes a metal salt containing hydration water, thespin rate was negative both in the case of using the driver and in thecase of using the iron, and it has been confirmed that the spin rate islower than in the case of Comparative Example 1, in which no water isincluded in the rubber composition.

Furthermore, in Examples 1 to 5, the durability performance is as low as10% or less, and it has been confirmed that the durability issignificantly higher than in Comparative Example 2, in which water wasincluded in the rubber composition in manufacturing the core.

All examples and conditional language provided herein are intended forpedagogical purposes of aiding the reader in understanding the inventionand the concepts contributed by the inventors to further the art, andare not to be construed as limitations to such specifically recitedexamples and conditions, nor does the organization of such examples inthe specification relate to a showing of the superiority or inferiorityof the invention. Although one or more embodiments of the presentinvention have been described in detail, it should be understood thatvarious changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A golf ball comprising: a core; and a cover,wherein the core is formed of a material molded under heat from a rubbercomposition, the rubber composition including components (A) through(C), where the components (A) through (C) are (A) a base rubber, (B) anorganic peroxide, and (C) a metal salt containing hydration water. 2.The golf ball as claimed in claim 1, wherein a moisture content of (C)the metal salt containing the hydration water in a molecular formulathereof is 6% by mass or more.
 3. The golf ball as claimed in claim 1,wherein a dissociation rate of water of (C) the metal salt containingthe hydration water in a case of heating (C) the metal salt containingthe hydration water up to 155° C. is 60% by mass or more.
 4. The golfball as claimed in claim 1, wherein a JIS-C hardness difference obtainedby subtracting a center hardness of the core from a surface hardness ofthe core is 20 or more.
 5. The golf ball as claimed in claim 1, wherein(A) the rubber composition further includes 0.1 parts by mass or moreand 5 parts by mass or less of an organosulfur compound per 100 parts bymass of the base rubber.
 6. The golf ball as claimed in claim 1, whereinthe metal salt of (C) the metal salt containing the hydration water isan inorganic compound.